Facsimile system



May l1, 1943. w. R. KOCH 2,319,139

` l FACSIMILE SYSTEM Filed sept. so, 1940 Z @l i; ,W10/0 Y M7/75,6 l M65/Maf ,Pfff/VHB HMH/H52 /wrf AVAYA'A yal 77991"r Inventor Patented May 11, 1943 FACSIMILE SYSTEM Winfield R. Koch, Haddoneld, N. J., assigner to Radio Corporation of America, a. corporation of Delaware Application september 99, 1940, serial Np. 359,106

Claims.

' `This invention relates to facsimile systems, and more particularly to output amplifiers for facsimile printers.

The first requirement of a facsimile amplifier and the one most difficult to meet is that the lower frequency limit is noty some definite frequency as in an ordinary audio amplifier, but is zero `or direct current. If the image or copy being analyzed has a large black border, this border may require a number of solid black scanning lines to represent it and the signal passing through the amplifier may remain substantially constant for a number of seconds. An amplifier that did not include a direct current as a part of its frequency spectrum would not amplify such a 4signal properly. The upper frequency limit is much easier to meet, being generally much lower than that of the usual high quality audio frequency amplifier. A

The simplest solution of the facsimile amplier problem is a direct current amplifier in which the amplifier tubes are directly connected anode to succeeding `tube control electrode and pure resistance coupling used throughout so that a lower frequency limit vof zero is obtained. Such an amplifier, though extremely simple, hasvseveral serious disadvantages, which necessarily limits its use to specialized services. Drifting of adjustment, and fluctuations of supply voltage are main sources of trouble. l

Several forms of self-regulating direct current amplifiers have been used, but these are generally veryfdiiiicult to build or the initial adjustments are hard to make, or they do not regulate closely enough with very low inputl voltages which are usually present during the reception of facsimile signals.

, According to this invention, an amplifier is provided whose output is not seriously affected by iiuctuations in supply voltages or maladjustment. The signal is impressed on a control electrode The principal object of this invention is to provide a push-pull amplier whose frequency range extends down to zero frequency.

Another object of this invention is to provide a stable direct current amplifier circuit.

' Other and incidental objects lof the invention will be apparent to those skilled in the art from a reading ofthe following4 specification and an Y`inspection of the accompanying drawing in which Figure 1 is a block diagram showing one form of this invention,

Figures 2, 3, 4 and 5 are circuit diagram showing other forms of this invention, and

Figure 6 is a graphical illustration of the results obtained through the use of this. invention in any of its preferred forms. Y y. f v

Referring now in moredetail to Fig. 1, the employment of one form of this invention is shown for the reception of radio facsimile signals. A radio receiver I supplies a limiter amplifier 2 with a facsimile signalwhich may be of the type in which an amplitude modulated subcarrier is used. Such a sub-carrier has a frequency which, with its side bands, may be passed by the usual audio amplifier in a high quality radio receiver. For specific example, the subcarrier may have a frequency of 2500 cycles.

Limiter amplifier 2 is used to square up al1 signals and give the equivalent signal of a keyed tone. In addition a threshold control may beincluded in this limiter to reduce all background noises to z'ero. Thus the output will be either-on at full value or off entirely, and no signals between these two values will be used. A description of a radio facsimile transmitter designed to broadcast such a signal and'a more detailed description of a limiteramplifiermaybe found in an article entitled Facsimile transmission and reception by Maurice Artzt in Electrical Engineers Handbook published by John Wiley and Sons, Inc.

The signal is then fed between. anode3 and anode dof the full wave rectifyin'g tube 5 through the push-pull secondary Winding 6 of coupling transformer l. The center tap in the secondary winding 6 is connected to the controlelectrode 9 of the amplifier tube Il. The cathode I3 of the rectifying tube 5 is connected to the cathode VI5 of the amplifier tube II so that the incoming signal is fed between the control electrode 9 and cathode I5 of the amplifier tube II. `A resistor I'I andbypass condenser I9 'are connected between the control electrode 9 and the cathode I5 to allow the accumulated electrons on the control electrode 9 to leak back to the cathode I5 and to complete the path for the current from the rectifier I5. The cathode I5 of tube II is connected directly to the cathode 2l of amplifier tube 23. This cathode circuit is provided with a resistor 25 connected to ground. There then results a voltage drop across resistor 25 in accordance'withthe impressed signal voltagedue to the change in current drawn by the 'amplifier tube I I.

The resultingnchange in potentialacross resistor 25 due to the change in current iiowing therethrough causes the cathode 2| to change in potential in accordance with the impressed signal voltage. Control electrode 21 of tube 23 is maintained at a fixed potential with respect to ground by the B-I- power supply and resistors 29 and 3|, which resistors may be so chosen to provide a proper biason the control electrode 21 With respect to the mean potential of the cathode 2I. The push pull output signal is then taken from the anode 33 and anode 34 to supply the facsimile printer 35, which is driven by the motor drive 31. The motor is preferably kept in synchronism with the transmitter by using the same synchronizing signal as is used by the facsimile signal transmitter motor drive. The electron discharge devices II and 23 may take the form of three-element tubes or may be preferably of the plural electrode type including screen grids 39 and 4 I.

y Upon the reception of a facsimile signal by the radio receiver I, the limiter amplier v2 passes an amplitude carrier to transformer 1. When the amplitude of the sub-carrier signal is great, a large amount 'of rectiiied voltage is impressed between the control electrode 9 and the cathode I5 driving the control electrode 9 more negative with respect to the cathode I5, thus allowing only a relatively small amount of current to flow through the amplifier tube II. This relatively small amount of current produces a small voltage drop in the resistor 25 so that cathode I5 and cathode 2| are at a relatively small positive potential with respectto ground.

It will beremembered that control electrode 21 of amplifier tube-23 is maintained at a fixed potential with respect to 'ground so that during the period which the cathode 2l is at a relatively low positive potential'with respect yto ground, there will kbe a relatively small potential differencebetvveen 'the control electrode 21 and the cathode 2|,'causing a large current flow between the anode 33 and the cathode 2| of amplifier tube 23. This results in a'push-pull output voltage which is'required for driving the facsimile Yprinter 35.

.It is true that the 'large current fiowing through the'resistor 25 `because of the large current in tube 23 tends'toincreasethe voltage `drop across resistor25 such as to producea degenerative effect, but this degeneration is Vnot sufficiently large to be objectionable.

Like numerals refer to similar parts through- Aout the several figures.

Referring now to Fig. '2, another form of this invention is shown in which the modulated carrier input signal is fed through transformer 1 to vrectifier tube'5 whose cathodes 4U and 42 are connected to opposite ends of the secondary 6 of transformer 1. The center tap of the secondary 6 of transformer 'I is connected to the cathodes I5 and 2| of the amplifier tubes II and 23, respectively. In this form of the invention both anodes 3 4and 4 of the rectifier tube 5 are connected together and, in turn, connected to the control electrode 9 of amplifier tube' I I.

The circuit includes facsimile printer coils 43 and 45. One terminal ofthe printer coil 43 is connected to the output of amplifier tube I I while printer coil `45 is connected to the output of amplifier tube 23.

There is shown in Fig. 3 still another form of this invention in which the modulated sub-carrier is fed through transformer 1 to the anodes of the rectifier tube 5. The center tap in secondary 6 of transformer 1isagain connected to the control electrode 9 of amplifier tube I I. However, the cathode I3 of the rectier tube 5 is connected to ground through resistance 41. The cathode I3 is maintained at a slightly positive potential with respect to ground through a voltage drop in resistor 41 caused by its connection to the +B voltage supply through resistor 49. This slightly positive potential on cathode I3 is required in order to maintain the control electrode 9 of amplifier tube II at a negative potential With respect to its corresponding cathode I5 which, as previously described, .is maintained at a positive potential by a voltage drop across resistance 25.

It will benoticed that the principal difference between this Vform of the invention and those forms described above is that the rectified modulated carrier input signal is applied between the control electrode 9 of amplifier tube II and ground. It is true that increased degeneration is again experienced due to the drop in potential across resistor 25 in the cathode circuit of the amplifier tube II but this disadvantage iseasily overcome byincreasing the amplitude of the incoming signal.

Fig. 4 Yshows stillfanother form of this invention. The rectified'modulated carrier inputsignal'is applied between the VVcontrol electrode 9 of lamplifier tube II -and a variable tap on the cathode resistor 25. According to this form of the invention, cathode resistor 25 provides lthe necessary bias potential to the-control electrode 9 with respect to its corresponding cathode I5 in addition to allowing cathode 2| of amplifier tube 23 to change its potential in order to produce a current through tube 23 opposite in phase to that current through tube l I as previously described in more detail. Adjustment of the tap on resistance 25 may also be used to control the degeneration of the ycircuit which results from applying the voltage drop occurring across a portion of resistance 25 to the input circuit. Another fundamental difference between this form ofthe invention and those previously described is that an increase in the Aamplitude of the signal applied to the rectifier 'decreases the negative voltage on the control electrode 9.

Referring now to Fig-5, a further modification of this invention is obtained by the use of pushpull output electron discharge -devices 5I and 53. Tube 5Il contains aY cathode` I5, control electrode 9, a screengrid 55, suppressor grid 55, and Vanode 58. lTube 53 includes cathode 2|, control electrode 21, screen grid 51, suppressor grid 59 and lanode 6I. The screenjgrid 55 of tube 5I is connected to the anode l6I of tube 53 through resistance 63. Screen grid 51 of tube 53 is connected to the anode 58 of tube 5I through resistance 65.

As the controlelectrode 9 is made more negative in response to an increase in the amplitude of the 'modulated Vsub-carrier input signal, the plate current of the tube 5I is reduced. This causes less voltage drop through the printer` coil 43 causing the.screen'grid.51 of ,tube 53 .to become more positive. This, .in turn,v increases the plate current of tube 53, causingfmore drop through the printer coil 45 `sothat thescreengrid 55 of the tube 5I becomes less positive. This tends to further reduce the vplate vcurrent through tube 5I enhancing the yeiect of the signal voltage on the control electrode 9.

AIt is obvious `that withutubes having a. high mutual conductance and normal circuit constants, an unstable condition would exist causing one tube to quickly take a minimum oi currentl and the other tube a maximum. However, due to the degenerative effect of the cathode resistor 25 and to the voltage drop through the resistors 63 and 65 between the plates and connected screen grids of the opposing tubes, the operation of the circuit can be made very stable.

In order to secure the proper phase relations for the various signal components, a corrective element was added to compensate for the in'- ductive effect of the printer coils 43 and 45. In its simplest form, a satisfactory corrective circuit may be composed of small compensating capacities 6l and 69 connected between ground and screen grids 55 and 51, respectively.

While in a preferred form of this invention, resistances 63 and 65 in the plate and screen circuits are equal, it is obvious that unequal resistances may be employed if it is desirable to secure a modified operating characteristic.

Fig. 6 shows in curve a the current flowing through one of the printer coils such as coil 43, curve b shows the current owing through the other of the printer coils such as coil 45. The resultant effective current through both the printer coils 43 and 45 is represented by curve c. This indicates a relatively large amplitude range through which this amplifier circuit will faithfully reproduce an incoming facsimile image signal.

If a frequency modulated sub-carrier is used, this invention may, of course, be similarly employed. Transformer 'l would then be of the fre quency discriminator type in which a change in frequency of the sub-carrier would cause a correspon-ding change in the voltage applied between the control electrode 9 and its corresponding cathode I5.

While several systems for carrying this invention into effect have been indicated and described, it will be apparent to one skilled in the art that this invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of this invention as set forth in the appended claims.

I claim as my invention:

l. In a picture-transmitting system including a balanced output circuit and a pair of discharge devices each having an anode connected to said output circuit and each discharge device having a control electrode and aI cathode, the combination of a resistor having a terminal to which is connected the cathode of both of said discharge devices, a floating detector circuit for applying a signal between the control electrode of one of said discharge devices and said terminal, and means to normally maintain the control electrode of the other discharge device at a predetermined potential so that signal energy applied between said terminal and the control electrode of the first discharge device is also applied between the control electrode of the other of said discharge devices and said terminal through said resistor.

2. In a picture-transmitting system including a balanced output circuit and a pair of discharge devices each having an anode connected to said output circuit, an auxiliary electrode connected to the anode of the other of said discharge devices to cause the potential of each of said auxiliary electrodes to vary in phase with the variation of potential of the anodes in the other of the to normally maintain the control electrode ofk the other discharge device at predetermined potential so that signal energy applied between the" control electrode and said predetermined point on said resistor of the first discharge device is also applied between the control electrode of theY other of said discharge devices and said predetermined point on said resistor.

3. A signal amplifier having a balanced output" circuit, a pair of discharge devices each having an anode connected to said output circuit and'v each having a control electrode and a cathode,V a resistor common to both of said cathodes, a

circuit having no point of fixed potential for applying a signal between the control electrode of one of said discharge devices and a predetermined intermediate point on said resistor, and means to normally maintain the control electrode of the other discharge device at a predetermined potential so that signal energy applied between the control electrode and said predetermined point on said resistor of the first discharge device is equa-1 to that applied between the control electrode of the other of said discharge devices and said predetermined intermediate point on said resistor.

4. A signal amplifier having a balanced output circuit including a pair of discharge devices, each having an anode connected to said output circuit and each having a cathode, a control electrode and at least one auxiliary electrode, a resistor common to both cathodes, means for applying a signal between the control electrode of one of said discharge devices and a predetermined point on said resistor, means to maintain the control electrode of the other discharge device at a predetermined potential so that signal energy applied between the control electrode and said point on said resistor of the rst discharge device is substantially equal to that applied between the control electrode of the other of said discharge devices and said point on said resistor, and a connection including resistance between said auxiliary electrode and the anode of the other of said discharge devices to cause the potential of each of said auxiliary electrodes to vary in phase with the variation of potential of the anodes in the other of the said discharge devices.

5. A signal amplifier comprising in combination a pair of discharge devices each having a cathode, a control electrode, an anode and an auxiliary electrode, means for applying a signal between the control electrode and the cathode of one of said discharge devices, means for causing said cathodes to change their potential in accordance with said signal, means for maintaining the v 

